Self-aligned silicides, commonly referred to as salicides, are formed when metal is deposited over an MOS structure and reacted with exposed silicon areas such as the source and drain, as well as exposed polysilicon areas on the gate, to form silicides. Oxide structures along the gate, known as oxide spacers, prevent the gate and source/drain areas from being electrically connected by avoiding silicide formation on the oxide. Following the silicide formation, a selective etch is used to remove unreacted metal without attacking the silicide.
The most widely used silicide for the salicide process is TiSi.sub.2, although platinum and molybdenum silicides are also employed. Titanium silicide is attractive for this application because it exhibits the lowest resistivity of the refractory metal silicides. Since it can reduce native oxide layers, it is the most widely accepted refractory metal for reliably forming a silicide on both poly- and single-crystalline silicon by thermal reaction.
In the salicide structure, the silicide is formed in the diffusion areas and on the polysilicon gate. The oxide spacers separate these two regions by only about 2,000-3,000 .ANG.. Thus, any lateral formation of silicide can easily bridge this separation and cause the gate to become shorted to the source/drain. It has been observed that if titanium silicide is formed by conventional furnace annealing in an inert gas atmosphere, such lateral titanium silicide formation rapidly occurs. By annealing in an ambient of nitrogen, the titanium absorbs a significant amount of nitrogen and, at the same time, reacts with the nitrogen and forms a nitride phase on the titanium surface. Once the titanium is fully reacted with the nitrogen, lateral silicide reaction is essentially suppressed.
Unfortunately, during the annealing process silicon can diffuse from the source, drain and gate regions and into the titanium which covers the oxide region. Thus, titanium silicide can be formed over the isolation oxide and sidewall spacers, as well as over the source, drain and gate regions. This silicide is not etched away by the selective etch. Therefore, when the second high-temperature anneal is performed, the silicide is converted to the low resistivity C54 phase. If this low resistivity silicide provides a continuous electrical path between the source, drain and gate regions, then the device will fail.
These problems can be reduced by controlling the rate of nitridation of the titanium. The titanium must be nitrided quickly enough to suppress the silicide formation over the oxide, yet slowly enough to permit sufficient silicide to form in the source, drain and gate regions. This is a very delicate process. The process can be better controlled by limiting the thickness of the sputtered titanium. This allows the titanium over the oxide to be quickly nitrided. Once it is nitrided, it cannot be silicided. However, if the sputtered titanium is too thin, then the thickness of the silicide formed over the source, drain and gate regions will be insufficient to provide low resistance.